What happened to my legacy add-ons on AMO and where to get them now?
Classic Theme Restorer supports Firefox, Waterfox and Basilisk browser versions 45.x-56.x! (No support for Fx 57+ or Wf 68+!)
Users of Debian 9 or later and Ubuntu 16.04 or later: apt-get install xul-ext-classic-theme-restorer
This add-on restores squared tabs, the application button with application menu (known from Firefox for Windows), the add-on bar, bookmarks/history menu/sidebar buttons and other buttons. It moves star & feed buttons into location bar, adds status bar element to the ui and allows many items to be movable. All features are fully optional. Some buttons have to be moved to toolbars manually using browsers 'customizing' mode.
Main features (on options window)
Features (not on options window)
Squared tabs Default 'curved tabs' can be optionally replaced with classic 'squared tabs'. Tabs on MacOSX look more like Windows tabs with MacOSX colors on purpose.
Curved alternative tabs All tabs are curved and use colours known from squared tabs.
Tabs 'not' on top Simulates 'tabs.onTop=false' setting and moves tabs below navigation toolbar. On MacOSX tabs also get a 'reversed appearance'.
Application button / application menu settings
Application button on [Firefox] titlebar [Windows only] The titlebar button is fixed on titlebars left like on Firefox since version 4.0. It is enabled by default on Windows systems.
Application button on toolbars On toolbars the button appears like a default toolbar button.
Application button colours On titlebar and tabs toolbar the application button can be orange, blue (Aurora), black (Nightly), blue (Palemoon), red, green and gray in normal browsing mode and purple in private browsing mode like on previous Firefox versions.
Note The new appmenu is not an exact copy of the old one, but looks as original as possible. The 'splitmenu' will not be restored and the appmenu popup/submenus also miss some non-replicable menuitems.
Small navigation toolbar buttons Navigation toolbar buttons get alternative margins and paddings to simulate old 'small icon view'.
Hide 'Navigation Toolbar' Hiding navigation toolbar might be interesting after content got moved to other toolbars.
'Add-on Bar' A toolbar at the bottom of the window. All movable buttons and items can be placed on it. Add-on bar can be enabled/disabled using toolbars context menu (right-click on a toolbar) or using 'Ctrl + /' (Windows/Linux) btw. 'Cmd + /' (MacOSX).
'Additional Toolbar' A toolbar between navigation toolbar and bookmarks toolbar. It can be enabled/disabled using toolbars context menu (right-click on a toolbar).
Icons + text / text only mode Simulates old icons + text / text modes where text was displayed below toolbar buttons or button icons got hidden.
Bookmarks menu button A default 'menu button' which opens the 'classic' bookmarks menu. Positioned on bookmarks toolbar the button also displays its label.
Bookmarks and history sidebar buttons Buttons which toggle bookmarks and history sidebar.
Back-forward, reload and stop buttons Browsers default buttons got merged with urlbar and cannot be moved to different toolbar positions or to different toolbars. CTR adds duplicates of these buttons to the customization palette.
Combined reload-stop button Place reload button AFTER stop button and enable the corresponding option on CTRs preference window. Hovering the option on preference window to get extra information. Toolbar: [something]...[stop-button][reload-button]...[something]
Spaces, flexible spaces and separators These toolbar items appear in customizing mode and can be moved to toolbars like on previous Firefox versions.
Windows - Linux - MacOSX This add-on was tested on Windows 7 (AeroBasic, AeroGlass, Classic) & 8(.1) & XP (Classic, Luna-Blue/Silver/Olive, Zune, Royale), Linux Mint KDE, Ubuntu and MacOSX 10.8/10.9. On other Systems or non OS-default themes tab colors may look wrong. Bookmarks, history, stop, reload, back and forward toolbar buttons use default OS icons (Firefox Windows icons on Windows systems, Firefox Mac icons on MacOSX and Firefox Linux icons on Linux systems).
Major problems with (discontinued?) Foxtab and Shareaholic add-ons It is strongly recommended to remove Foxtab and Shareaholic before installing Firefox 29 or newer, because both break Firefox ui in Fx29+. Using Foxtab and/or Shareaholic with CTR breaks the ui even more. If one of these add-ons already has corrupted the ui, the add-ons manager has to be accessed via menubar (right-click on a toolbar -> select menubar, Tools -> add-ons) and CTR, Shareaholic and Foxtab have to be uninstalled. After Firefox has been restarted, "restore defaults" in customizing mode has to be hit once. CTR can be reinstalled afterwards and will work properly.
Info, known 'issues', 'glitches'
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It is recommended to switch to a clean new profile to avoid UI issues after upgrading from Firefox 28 (and older) to Firefox 29 (and newer). Profile Switcher can help too.
Note: Triggering "Restore Defaults" in customizing mode to reset toolbar configuration is enough in most cases and much easier to perform.
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AVAST AV software,"Browser Cleanup" and disappearing Firefox add-ons
Current Avast versions offer a bugged version of "Browser Cleanup", which accidentally removes Firefox add-ons without any proper reason! Although Avast is currently working on fixing this issues, it is highly recommended to remove or disable "Browser Cleanup" until problems are fixed.
Look at gHacks, Mozillazine, Bugzilla or Avast forums for more info.
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gHacks.net article (10 Reasons why you may want to try Firefox’s Classic Theme Restorer add-on)